1. Technical Field
This invention relates generally to power control in a wireless communication device transmitter, and, more particularly, to a system for allowing a time division multiple access (TDMA)/code division multiple access (CDMA) portable transceiver to operate using a closed loop power control feedback system.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes where various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards, to which these systems must conform have evolved. For example, in the United States, portable communications systems complying with the IS-136 standard specify the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8QPSK), offset π/4 differential quadrature phase shift keying (π/4-DQPSK) or variations and the access format is time division multiple access (TDMA). Other standards may require the use of, for example, code division multiple access (CDMA).
Similarly, in Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrowband TDMA access environment. Although there are many differences that distinguish systems complying with IS-136 from systems complying with GSM, a substantial difference is the manner in which transmit power is controlled. In IS-136 systems, the transmit power control is achieved using a reference voltage control signal supplied to the portable transceiver from the base station with which the portable transceiver is communicating. In this “open loop” feedback control system, the base station sends a reference signal requesting that the portable transceiver either increase or decrease transmit power. In response, the portable transceiver will comply by adjusting the transmit power in pre-determined steps of, for example, 2 dB. The base station continues sending the power control adjustment signal until the portable transceiver is supplying an appropriate transmit power level.
In contrast to portable communication systems complying with the IS-136 standard, those systems complying with the GSM standard have a much stricter transmit power control requirement. Instead of having to adjust transmit power in relatively large steps, systems complying with GSM are required to set transmit power at a particular level, with very little variation. In order to achieve this strict transmit power control requirement, systems complying with GSM implement a closed feedback loop power control system. The closed feedback loop power control system measures a portion of the transmit power, compares this transmit power with a reference power level developed within the portable transceiver to develop an error signal and the resulting error signal is fed back to the input of the power amplifier to adjust the transmit power to comply with the reference power level. Importantly, the closed feedback loop resides completely in the transmit portion of the portable communication device.
Furthermore, in a typical GSM mobile communication system using narrowband TDMA technology, a GMSK modulation scheme supplies a very clean phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a non-linear power amplifier, which is highly efficient, can be used thus allowing efficient modulation of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Other transmission standards, such as those employed in IS-136, however, use a modulation scheme where both a PM signal and an amplitude modulated (AM) signal are transmitted. Standards such as these increase the data rate without increasing the bandwidth of the transmitted signal. Unfortunately, existing GSM modulation schemes are not easily adapted to transmit a signal that includes both a PM component and an AM component. One reason for this difficulty is that in order to transmit a signal containing a PM component and an AM component, a highly linear power amplifier is required. Unfortunately, highly linear power amplifiers are very inefficient, thus consuming significantly more power than a non-linear power amplifier and drastically reducing the life of the battery or other power source.
This condition is further complicated because transmitters typically employed in GSM communication systems transmit in bursts and must be able to control the ramp-up and ramp-down of the transmit power as well as have a high degree of control over the output power level over a wide power range. As mentioned above, in GSM this power control is typically performed using a closed feedback loop in which a portion of the signal output from the power amplifier is compared with a reference signal and the resulting error signal is fed back to the input of the power amplifier.
When attempting to include a PM component and an AM component in a GSM type modulation system, the power control loop will tend to fight against the amplitude variations present in the signal while attempting to maintain the desired output power. In such an arrangement, the power control loop tends to cancel the AM portion of the signal.
In systems that transmit signals containing both PM and AM components, the output power can be controlled by setting a calibrated control signal on the power amplifier. Unfortunately, this requires the use of a highly linear, and therefore very inefficient, power amplifier. In non-burst transmission systems the output power may be controlled by a feedback loop having a time-constant that is very low compared to the time-constant of the amplitude variations of the modulator. Another known method to control the output power is to “pre-distort” the modulated signal in such a way that the power control loop will cancel the effect of the pre-distortion. In such a method, the amplitude information is passed through a transfer function that is the inverse of the power control loop transfer function. Unfortunately, these methods are costly and inefficient.
In systems complying with IS-136, transmit power control is achieved via an open loop feedback path in which the base station sends a reference power signal to the portable transceiver. In such a system, there is no specific power requirement, just the command to either increase or decrease power output. This is known as an open loop power control system.
In those transmission standards where both a PM signal and an AM signal are sent to a power amplifier, unless the power amplifier is very linear, it may distort the combined transmission signal by causing undesirable AM to PM conversion. This conversion is detrimental to the transmit signal and can require the use of a costly and inefficient linear power amplifier.
With the increasing desirability of developing one worldwide portable communication standard, it would be desirable to allow portable transceivers that comply with the IS-136 standard, and provide both a PM component and an AM component to the transmit signal to operate in a GSM system in which there is typically only a PM component to the transmit signal. Furthermore, as the GSM standard evolves further, such as with the development of enhanced data rates for GSM evolution (EDGE), it is desirable to have one portable transceiver that may operate in all systems. Unfortunately, because the GSM system requires closed loop power control, it is currently not possible to use a portable transceiver complying with the IS-136 standard in a GSM/EDGE mobile communication environment.